1. Field of the Invention
The present invention relates to a color image signal generator used for a video game machine. More particularly, the present invention relates to a color image signal generator arranged to generate red, green and blue image signals for each pixel in accordance with red, green and blue image data and with brightness data. The present invention also relates to a storage medium for storing programs to control such a color image signal generator.
2. Description of the Related Art
A conventional video game machine may incorporate many electronic components and units for enabling a player (or players) to enjoy a video game by the video game machine. Among these units may be a color image signal generator arranged for generating red, green and blue image signals (analog signals) based on red, green and blue image data supplied from a look-up table incorporated in the video game machine.
Typically, the look-up table may be made up of a RAM (random access memory). The red, green and blue image data from the look-up table are converted into corresponding red, green and blue analog signals by digital-to-analog converters (D/A converters).
The analog image signals from the color image signal generator are supplied to a display unit. As a result, desired characters (each of which is made up of plural "objects") are visually presented on the screen of the display unit.
Disadvantageously, the memory of the conventional look-up table has not been put to effective use. Specifically, the bit number of a memory in general is eight (8) bits or sixteen (16) bits. Color image data are made up of three kinds of data, namely, red, green and blue image data. Thus, supposing that the look-up table is of 16-bit and five (5) bits are used for each color, one bit in the look-up table remains to be unused.
For avoiding the above problem, the applicant of the present application once proposed a color image signal generator shown in FIG. 6 of the accompanying drawings. The illustrated signal generator includes a look-up table 51, buffers 52Ra-52Rd, 52Ga-52Gd and 52Ba-52Bd, three-state gates 53Ra-53Rd, 53Ga-53Gd and 53Ba-53Bd, amplifiers 54R, 54G and 54B, and resistors Rra-Rrh, Rga-Rgh and Rba-Rbh.
The resistors Rra, Rga and Rba may have a resistance of 1600.OMEGA., the resistors Rrb, Rgb and Rbb may have a resistance of 800.OMEGA., the resistors Rrc, Rgc and Rbc may have a resistance of 400.OMEGA., the resistors Rrd, Rgd and Rbd may have a resistance of 200.OMEGA., the resistors Rre, Rge and Rbe may have a resistance of 800.OMEGA., the resistors Rrf, Rgf and Rbf may have a resistance of 400.OMEGA., the resistors Rrg, Rgg and Rbg may have a resistance of 200.OMEGA., and the resistors Rrh, Rgh and Rbh may have a resistance of 100 .OMEGA..
The look-up table 51 outputs 4-bit red image data, 4-bit green image data and 4-bit blue image data in parallel. The 4-bit red image data are supplied to the buffers 52Ra-52Rd. Similarly, the 4-bit green image data are supplied to the buffers 52Ga-52Gd, while the 4-bit blue image data are supplied to the buffers 52Ba-52Bd.
The look-up table 51 also outputs 4-bit brightness data together with the three color image data. The brightness data are supplied to the three-state gates 53Ra-53Rd, 53Ga-53Gd and 53Ba-53Rd via their control terminals. As is shown, the input terminals of the respective three-state gates 53Ra-53Rd, 53Ga-53Gd and 53Ba-53Bd are grounded.
It is now supposed that the combination of the four resistors Rra-Rrd in parallel has an equivalent resistance of R1, while the combination of the four resistors Rre-Rrh in parallel has an equivalent resistance of R2. In this instance, the output voltage from the buffers 52Ra-52Rd is divided into two lower voltages in accordance with the ratio of R1 to R2, and only one of the lower voltages is supplied to the amplifier 54R. The supplied data are amplified by the amplifier 54R and then output as analog red image signals.
Similarly, analog green image signals are obtained from the amplifier 54G, while analog blue image signals are obtained from the amplifier 54B.
According to the color image signal generator shown in FIG. 6, the look-up table 51 can be used effectively. Supposing that the look-up table 51 is a 16-bit memory, 12 bits may be used for the three kinds of image data (4 bits for each color) and the remaining 4 bits may be used for the brightness data. In this way, it is possible to use all of the 16 bits of the look-up table 51, thereby enabling provision of sixty five thousand five hundred and thirty six (65536) colors in total.
However, the color image signal generator of FIG. 6 has been found disadvantageous in the following points.
Specifically, the signal generator of FIG. 6 is an analog circuit using the resistors Rra-Rrh, Rga-Rgh and Rba-Rbh for example. In general, analog circuits tend to have constraints on designing freedom. Further, it is difficult to generate accurate color image signals with the use of an analog circuit. Referring to the signal generator of FIG. 6, the buffers 52Ra-52Rd, 52Ga-52Gd and 52Ba-52Bd are TTL (transistor-transistor-logic) circuits. In this case, the high-level output voltage from these buffers will unfavorably vary due to variations in load. Thus, it becomes difficult to maintain an accurate image signal level in accordance with the combination of the image data and the brightness data.
Still further, when the resistances of the resistors Rra-Rrh, Rga-Rgh and Rba-Rbh are increased, the delay time in signal transmission will become longer. On the other hand, when the above resistances are decreased, the power consumption of the color image signal generator tends to become unduly larger or the input voltage to the amplifiers 54R, 54G and 54B may decrease. Additionally, in the signal generator of FIG. 6, the variable range of image signals based on the brightness data is unfavorably limited, so that it cannot be expected to obtain satisfactory results of fade-in or fade-out.